Actually check backends are alive in haproxy

This adds the simplest form of health checking to haproxy, a tcp check
to the backends. We can do more sophisticated checks like checking ssl
negotiates or even HTTP requests but for now this is probably a good
improvement.

Change-Id: I3c6b07df4b3e0c380c757e1e5cb51ae0be655f34

playbooks/roles/haproxy/templates/haproxy.cfg.j2

@@ -27,9 +27,10 @@ listen {{ listener.name }}
   mode tcp
   balance source
   option tcplog
+  option tcp-check
 
   {% for server in listener.servers %}
-  server {{ server.name }} {{ server.address }}
+  server {{ server.name }} {{ server.address }} check
   {% endfor %}
 
 {% endfor %}